Direct infection of pulmonary macrophages with influenza virus in vitro does not alter macrophage functions necessary for staphylococcal clearance. To determine whether these functions are altered during viral pneumonitis, we evaluated macrophages recovered from influenza-infected mice which had undergone aerosol challenge with Staphylococcus aureus. Sublethal infection with influenza A/PR8 produced patchy hemorrhagic pneumonia in CF1 mice and significantly reduced the intrapulmonary killing of staphylococci inhaled during aerosol challenge. However, only a small fraction of macrophage monolayers established from animals with influenza expressed viral hemagglutinin on their plasma membrane, and alveolar macrophages from infected mice ingested staphylococci and yeast cells in vitro at the same rate as control macrophages. The in vitro intracellular bactericidal activity against staphylococci ingested in vivo was comparable in monolayers from control and PR8-infected mice. In experiments with more severe influenza infections (mortality greater than 50% by day 7), a larger fraction of the staphylococci recovered by bronchoalveolar lavage appeared to be ingested in vivo during the aerosol exposure in the PR8-infected mice than in the control mice, but intracellular killing by macrophages during in vitro incubation was similar in control and PR8 monolayers. Hence, the severity of viral infection did not influence intracellular bactericidal activity. In virus-infected mice, a significantly larger fraction of viable staphylococci in the lower respiratory tract at the end of aerosol exposure was adherent to the trachea and major bronchi. In summary, PR8 infection established by intranasal inoculation impaired staphylococcal killing in the lung even though these infections did not alter in vivo ingestion rates or in vitro intracellular killing rates of macrophage populations in bronchoalveolar spaces.